She has all the documents, keys paperwork etc. She wanted him to sell it for her but never expected him to disappear once it was sold. She said she was at work when it happened.

Sounds incredibly dodgy,very convenient for the boyfriend to go awol,when any income that the now bankrupt woman receives needs to be declared.

Has she (bankrupt) reported it to the Police,if not, I suggest you or your sister does.
To my mind the boyfriend has stolen the money from your sister, as the car was not his to sell,and is now worth nothing as the car belongs to Lombard,something I'm sure the boyfriend knew!